Linq 子查询（SELECT 中的 SELECT）答案

【问题标题】：Linq Sub Query (SELECT in SELECT)Linq 子查询（SELECT 中的 SELECT）
【发布时间】：2011-12-15 01:19:07
【问题描述】：

我在 MySQL Entity 中遇到了本机连接子查询问题。 SQL 代码是；

数据库结构是；

tbl_urunler_kod
id、kod
- 1、pkod1
- 2、pkod2
- 3、pkod3

tbl_urunler
id、kod_id
- 1, 1
- 2, 2
- 3、3
- 4、空
- 5、空
- 6、空
- 7、空
- 8，空
- 9，空

SELECT (SELECT k.kod FROM tbl_urunler_kod k WHERE k.id=t.kod_id) AS kod FROM tbl_urunler t

它给出了 9 条记录。

我试试这个，

 var SQL = (from p in SME.tbl_urunler
                   from k in SME.tbl_urunler_kod
                              where k.id == p.kod_id
                   select new
                   {
                       kod = k.kod
                   });

它只返回 3 条记录。因为 tbl_urunler_kod 只有不为空的 3 条记录。其他 6 条记录为空。我想看所有的记录。我该怎么做。

我试试这个；

var SQL2 = (from p in SME.tbl_urunler
                       select new
                       {
                           kod = (from k in SME.tbl_urunler_kod where k.id == p.kod_id select new { k.kod })
                       });

它给出了这个错误；

'System.Data.Common.Internal.Materialization.CompensatingCollection`1[f__AnonymousType0`1[System.String]]' türündeki nesne 'System.String' türüne atılamadı。说明：执行当前 Web 请求期间发生未处理的异常。请查看堆栈跟踪以获取有关错误及其源自代码的位置的更多信息。异常详细信息：System.InvalidCastException: 'System.Data.Common.Internal.Materialization.CompensatingCollection`1[f__AnonymousType0`1[System.String]]' türündeki nesne 'System.String' türüne atılamadı。

感谢您的帮助。

【问题讨论】：

标签： linq entity-framework entity subquery

【解决方案1】：

为此，我创建了一个小模拟器。请看结果；

public class tbl_urunler_kod
{
    public int ID { get; set; }
    public string Kod { get; set; }
    public tbl_urunler_kod() { }
    public tbl_urunler_kod(int _ID, string _Kod)
    {
        _ID = ID;
        _Kod = Kod;
    }
}

public class tbl_urunler
{
    public int ID { get; set; }
    public string Adi { get; set; }
    public int KodID { get; set; }
    public tbl_urunler() { }
    public tbl_urunler(int _ID, string _Adi, int _KodID)
    {
        _ID = ID;
        _Adi = Adi;
        _KodID = KodID;
    }
}

public List<tbl_urunler_kod> L_tbl_urunler_kod = new List<tbl_urunler_kod>();
public List<tbl_urunler> L_tbl_urunler = new List<tbl_urunler>();

protected void Page_Load(object sender, EventArgs e)
{
    Fill();
    Result1();
    Result2();
    Result3();
    Result4();
    Result5(); //Its working
    Result6(); //Its working
    Result7(); //Its working
}

public void Fill()
{
    #region tbl_urunler_kod filling
    L_tbl_urunler_kod.Add(new tbl_urunler_kod { ID = 1, Kod = "Kod_1" });
    L_tbl_urunler_kod.Add(new tbl_urunler_kod() { ID = 2, Kod = "Kod_2" });
    L_tbl_urunler_kod.Add(new tbl_urunler_kod() { ID = 3, Kod = "Kod_3" });
    #endregion

    #region tbl_urunler filling
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_1", ID = 1, KodID = 1 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_2", ID = 2, KodID = 2 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_3", ID = 3, KodID = 3 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_4", ID = 4 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_5", ID = 5 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_6", ID = 6 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_7", ID = 7 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_8", ID = 8 });
    L_tbl_urunler.Add(new tbl_urunler() { Adi = "Urun_9", ID = 9 });
    #endregion
}

public void Result1()
{
    Response.Write("<h1>Result 1</h1>");

    var SQL = (from p in L_tbl_urunler
               from k in L_tbl_urunler_kod
               where k.ID == p.KodID
               select new
               {
                   p.Adi,
                   kod = k.Kod
               }); //result count = 3 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
    }

    Response.Write("<br/><hr/>");
}

public void Result2()
{
    Response.Write("<h1>Result 2</h1>");

    var SQL = (from p in L_tbl_urunler
               from k in L_tbl_urunler_kod
               where k.ID == p.KodID || p.KodID == 0
               select new
               {
                   p.Adi,
                   kod = k.Kod
               }); //result count = 21 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
    }

    Response.Write("<br/><hr/>");
}

public void Result3()
{
    Response.Write("<h1>Result 3</h1>");

    var SQL = (from p in L_tbl_urunler
               let LKod = L_tbl_urunler_kod.Where(k => k.ID == p.KodID).Select(x => x.Kod)
               select new
               {
                   p.Adi,
                   kod = LKod
               }); //result count = 9 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
        //System.Linq.Enumerable+WhereSelectListIterator`2[test+tbl_urunler_kod,System.String]
    }

    Response.Write("<br/><hr/>");
}

public void Result4()
{
    Response.Write("<h1>Result 4</h1>");

    var SQL = (from p in L_tbl_urunler
               select new
               {
                   p.Adi,
                   kod = (from k in L_tbl_urunler_kod where k.ID == p.KodID select new { k.Kod })
               }); //result count = 9 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
        //System.Linq.Enumerable+WhereSelectListIterator`2[test+tbl_urunler_kod,<>f__AnonymousType3`1[System.String]]
    }

    Response.Write("<br/><hr/>");
}

public void Result5()
{
    Response.Write("<h1>Result 5</h1>");

    var SQL = (from p in L_tbl_urunler
               select new
               {
                   p.Adi,
                   kod = L_tbl_urunler_kod.Where(k => k.ID == p.KodID).Select(x => x.Kod).FirstOrDefault()
               }); //result count = 9 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
    }

    Response.Write("<br/><hr/>");
}

public void Result6()
{
    Response.Write("<h1>Result 6</h1>");

    var SQL = (from p in L_tbl_urunler
               select new
               {
                   p.Adi,
                   kod = (from k in L_tbl_urunler_kod where k.ID == p.KodID select new { k.Kod }).FirstOrDefault()
               }); //result count = 9 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
    }

    Response.Write("<br/><hr/>");
}

public void Result7()
{
    Response.Write("<h1>Result 7</h1>");

    var SQL = (from p in L_tbl_urunler
               let LKod = L_tbl_urunler_kod.Where(k => k.ID == p.KodID).Select(x => x.Kod).FirstOrDefault()
               select new
               {
                   p.Adi,
                   kod = LKod
               }); //result count = 9 record;

    foreach (var i in SQL)
    {
        Response.Write(i.Adi + " - " + i.kod + "<br/>");
    }

    Response.Write("<br/><hr/>");
}

结果 1

Urun_1 - Kod_1
Urun_2 - Kod_2
Urun_3 - Kod_3

结果 2

Urun_1 - Kod_1
Urun_2 - Kod_2
Urun_3 - Kod_3
Urun_4 - Kod_1
Urun_4 - Kod_2
Urun_4 - Kod_3
Urun_5 - Kod_1
Urun_5 - Kod_2
Urun_5 - Kod_3
Urun_6 - Kod_1
Urun_6 - Kod_2
Urun_6 - Kod_3
Urun_7 - Kod_1
Urun_7 - Kod_2
Urun_7 - Kod_3
Urun_8 - Kod_1
Urun_8 - Kod_2
Urun_8 - Kod_3
Urun_9 - Kod_1
Urun_9 - Kod_2
Urun_9 - Kod_3

结果 3

Urun_1 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_2 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_3 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_4 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_5 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_6 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_7 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_8 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]
Urun_9 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,System.String]

结果 4

Urun_1 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_2 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_3 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_4 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_5 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_6 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_7 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_8 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]
Urun_9 - System.Linq.Enumerable+WhereSelectListIterator2[test+tbl_urunler_kod,<>f__AnonymousType31[System.String]]

结果 5

Urun_1 - Kod_1  
Urun_2 - Kod_2  
Urun_3 - Kod_3  
Urun_4 -  
Urun_5 -  
Urun_6 -  
Urun_7 -  
Urun_8 -  
Urun_9 -

结果 6

Urun_1 - { Kod = Kod_1 }  
Urun_2 - { Kod = Kod_2 }  
Urun_3 - { Kod = Kod_3 }  
Urun_4 -  
Urun_5 -  
Urun_6 -  
Urun_7 -  
Urun_8 -  
Urun_9 -

结果 7

Urun_1 - Kod_1  
Urun_2 - Kod_2  
Urun_3 - Kod_3  
Urun_4 -  
Urun_5 -  
Urun_6 -  
Urun_7 -  
Urun_8 -  
Urun_9 -

谢谢。

【讨论】：

【解决方案2】：

你试过了吗：

var SQL = (from p in SME.tbl_urunler
               from k in SME.tbl_urunler_kod
                          where k.id == p.kod_id || p.kod_id == null
               select new
               {
                   kod = k.kod
               });

更新以下是我从您的 cmets 中可以理解的内容：

var q = from u in SME.tbl_urunler
    join k in SME.tbl_urunler_kod on u.kod_id equals k.id into JoinedUK
    from k in joinedUK.DefaultIfEmpty()
    select new
    {
        id = u.id,  
        kod = k != null ? k.kod : null
    }

【讨论】：

它非常可爱的解决方案。但它只解决这个空字段而不是空字段。但它已经得到了另一个记录。我必须总是添加一些值 SQL 语句。例如； k.id == p.kod_id || p.kod_id == null || p.kod_id == bla bla || p.kod_id == null bla bla
@AhmetYETİK：我不明白你的问题。如果要返回所有 p.Kod_id，则无需设置 where 子句。根据您的评论，您应该删除where k.id == p.kod_id || p.kod_id == null。否则，p.kod_id 要么等于 k.id 要么为 null。
对不起我的语言。我想尝试用另一种方式来解释它；这句 where (k.id == p.kod_id) - 返回 3 where (k.id == null) - 返回 0 where (k.id != null) - 返回 27 where (k.id == p.kod_id && p.kod_id ==null) - 返回 0 它有一些问题。因为它加入了 tbl_urunler 和 tbl_urunler_kod ...但是它会在 tbl_urunler 返回一行时查看 tbl_urunler_kod 并返回 kod。谢谢。
@AhmetYETİK：更新了我的答案
@AhmetYETİK：对不起，我忘了使用equals。请再次运行我更新的查询。

【解决方案3】：

这是 LingtoSql lambda 表达式

    var query = SME.tbl_urunler.Where
                (x=>x.tbl_urunler_kod.id == x.kod_id  || 
                x.kod_id == null).Select(x=>new {x.id})

【讨论】：

感谢您的回答@esi。我会在晚上尝试。我喜欢 lambda 语句。它更容易理解。 :)
Hello Ahmet lambda experssion 在 linq 上非常好，所以尝试学习它。
亲爱的@esi;但它只解决这个空字段而不是空字段。但它已经得到了另一个记录。我必须总是添加一些值 SQL 语句。例如; k.id == p.kod_id || p.kod_id == null || p.kod_id == bla bla 是否有任何其他通过 Lambda 的全局解决方案。再次感谢。
Ahmet 我无法清楚地理解您的评论。但是如果我没有弄错这很简单，您可以在 lambda experssion 中的 where 语句上使用任何变量，例如（ int count= 100; dataContext.TableName.Where(x=>x.id == count); )
我现在正在查看您的代码。它的加入很好。但它会在 tbl_urunler 返回一行时查看 tbl_urunler_kod 并返回 tbl_urunler 列并给出“kod”。它应该是这样的； (from p in SME.tbl_urunler select new { kod = (from k in SME.tbl_urunler_kod where k.id == p.kod_id select new { k.kod }) }); 谢谢。